The invention is directed to a method for the correct, i.e. visually correct, identification of the color information of colored scenes or image originals--referred to below as "color analysis" which derives color value signals pixel-by-pixel according to the multispectral method. The signals acquired according to this method should thereby be of such a nature, in conformity with the definition, that they describe the informational content "color" of an image for a person having normal chromatic vision such that color image reproduction systems can be driven directly or indirectly, for example after suitable conversion, with the assistance of these signals. These color image reproduction systems then reproduce the images such that a normal observer cannot distinguish them from the original.
The problems involved as well as the current state of the art in this field are presented in detail in Letters Patent DE 28 44 158 C3 "Verfahren zur Reproduktion von Originalvorlagen welche bezueglich ihres Farbgehaltes nach einem". The problems of current color image scanning systems deriving from the prior art shall be set forth first below. Subsequently, the properties and advantages of the multispectral method of the invention shall be set forth, compared to the three-region method previously exclusively employed in color analysis.
The laws of colorimetry supply the theoretical foundations for the proper, chromatically correct reproduction of image originals. These make it possible to unambiguously describe color perceptions with the assistance of only three values, even though the actual physical cause of the color as a sensory experience is electromagnetic radiation in the visible range of the spectrum from 380 through 780 nm. The reason for this enormous data reduction occurring in the visual perception process is the structure and the functioning of the human eye, as well as the subsequent processing of the primary information in the brain. This process is formulated in the simplest and clearest way by the fundamental colorimetric law, M. Richter, Einfuehrung in die Farbmetrik, 2nd Edition, Berlin, N.Y., de Gruyter, 1980, in accordance with which the eye linearly and steadily evaluates the incident radiation according to three mutually independent, spectral performance functions, whereby the individual effects added up to form an inseparable overall effect--the color sensation. The biological condition for this is that three types of light-sensitive sensory cells having different spectral sensitivities exist in the human eye. These three so-called fundamental spectral value curves are shown in FIG. 1.
The nature of the integral information processing with three different, spectral performance functions, however, results therein that a theoretically infinite plurality of different spectral color stimuli can trigger the same color sensations. Colors having the same appearance despite a differing spectral composition are referred to as metamerically or conditionally identical. A technical reproduction system which wishes to chromatically reproduce the color originals true to the original must, just like the human eye, consequently be able to recognize metameric colors as the same. On the other hand, it cannot distinguish specific colors from one another which the human eye recognizes as different. It thus "sees" certain colors more or less incorrectly and can then likewise no longer correctly reproduce them in the color synthesis. An exact correction of the electrical signals is not possible. A subsequent correction is possible only in certain exceptional instances. This means that color reproduction systems must have their design features based both on the biological givens of the human eye as well as on the further processing of the primary information in the brain. In terms of information theory, these are sync-oriented systems. Reproduction systems therefore fundamentally resolve into color analysis, signal processing, and color synthesis in accordance with FIG. 2. The color analysis under consideration here is comparable to color measurement. According to DIN 5033 there are three possibilities for color measurement:
1. the equality method PA0 2. the three-range method PA0 3. the spectral method. PA0 a) due to non-uniform illumination over the surface of the image to be scanned, PA0 b) due to an unknown spectral distribution of the illuminating light source, PA0 c) due to spectral-dependent sensitivities of the sensor elements, PA0 d) due to spectral-dependent and imaging-dependent transmission properties of the optical components in the beam path automatically and in interrelationship on the basis of a simple calibration event; and
The equality method is a purely subjective measurement method wherein the observer compares unknown colors to a collection of known colors, for example in a color atlas, and visually identifies them. This is not suitable for color analysis in color image reproduction.
The state of the art in color image processing is the three-range method. In technical systems, work is carried out exclusively according to this method. Analogous to the receptor rods in the eye, optoelectronic transducers are employed as light-sensitive sensors, these optoelectronic transducers converting the incident radiant power in the visible range of the spectrum into electrical signals. Dependent on the embodiment and arrangement of the sensors, one distinguishes:
a) picture element sensors that only cover the information of one picture element, for example photocells, photomultipliers and photodiodes. In order to acquire the entire information, for example, the luminescent spot of a cathode ray tube having an extremely short persistence time is conducted in raster-like fashion across the image with a suitable optics. The signal at the output of the photomultiplier is a measure for the remissivity or transmissivity of the picture element that has just been illuminated. Apparatus working according to this principle are employed in television technology for scanning moving picture films and diapositives (FIG. 3--flying spot scanner). In other apparatus having sensors of this type, for example, the image to be scanned is moved in raster-like fashion in a stationary light beam. Typical apparatus are drum scanners which are employed with preference in reprographics. The image to be scanned is clamped on a rotating drum and the optoelectronic system composed of light source, imaging system, and sensor is translationally conducted past the drum, so that the image is scanned in the form of a helical line (FIG. 4, drum scanner).
b) Sensors arranged in line-like fashion, what are referred to as CCD lines, are composed of a plurality of individual photodiodes (for example, 512, 1024, 2048 or 4096 photodiodes). The picture element informations of an entire image line are adjacent and parallel to one another, and are converted into sequential data streams. The image to be scanned must be moved perpendicular to the line direction. Typical apparatus that employ this sensor arrangement are flat bed scanners, as is known from the Desktop Publishing field (FIG. 5--flat bed scanner).
c) Planarly arranged sensors are, for example, the individual, mosaic-shaped elements of the storage target (light-sensitive semiconductor photolayer) in conventional video pick-up tubes or, on the other hand, the discrete photodiodes in modern CCD arrays arranged in groups of 512.times.512, 1024.times.1024, 2048.times.2048 or 4096.times.4096. They simultaneously acquire the entire image information and convert it into chronologically continuous image signals on the basis of suitable, electronic measures. In terms of structure, the planarly arranged sensors are most similar to the retina of the eye; their great advantage is comprised therein that no mechanical motions are now required for scanning an image.
Analogous to the structure of the retina, three types of light-sensitive sensors having three different spectral sensitivities are required in the three-range method as well, these being realized in technical apparatus in that either three different, optical correction filters are inserted in front of a sensor in rapid succession or, on the other hand, in that the optical beam path is split into three channels by beam splitters such that three sensors each having respectively one correction filter are present. There are also flat bed scanners without correction filters having only one CCD line, whereby the individual image lines are successively illuminated with a light source emitting in the red, green and blue spectral range (FIG. 5). So that technical reproduction system can then correctly implement the color analysis according to the three-range method, the effective spectral channel sensitivities must coincide with the three spectral sensitivities of the cone of the human eye according to FIG. 1 in a suitable way. As a result of the linear and steady color stimulus or trichromatic matrix (fundamental colorimetric wall), an exact color analysis can be implemented with all spectral sensitivity curves that can be linked via a linear transformation with the fundamental spectral value curves. It is thus not required and not even expedient for fabrication-oriented reasons and reasons of the signal-to-noise ratio to strive for a direct simulation of the fundamental spectral value curves since, in particular the p(.lambda.) and the d(.lambda.) curves lie extremely close to one another and thus have no good color separation properties. Even slight errors of the spectral sensitivity of the channel would lead to unbearably great errors in the color value signals, just as would signal noise or quantization errors.
According to the theory of additive color mixing, the spectral value curves or color mixing curves belonging to the referenced stimuli for real primary color, for example red, green and blue, can be identified for these real primary colors. These spectral value and color mixing curves in turn have to be a linear combination of the fundamental spectral value curves. FIG. 6 shows the spectral value curves r(.lambda.), g(.lambda.) and b(.lambda.) that are based on the spectral primary colors R=700 nm, G=546.1 nm and B=435.8 nm defined by the CIE (Commission International de I'Eclairage) in 1931. The CIE spectral value curves are linked with the fundamental spectral value curves (the spectral sensitivities of the three types of cone in the eye) by the following, linear equation system: ##EQU1##
FIG. 7 shows the spectral value curves of color television. They refer to the receiver primary colors R.sub.e, G.sub.e and B.sub.e of the picture screen luminescent phosphors defined by the EBU (European Broadcasting Union). They, too, are linearly linked to the fundamental spectral value curves.
Color mixing curves for real primary colors corresponding to FIGS. 6 and 7 have significantly more beneficial color separation properties than the fundamental spectral curves according to FIG. 1. Dependent on the selection of the referenced stimuli, however, they comprise more or less great, negative components of the spectral sensitivity that cannot be technically realized in this way with sensors. In apparatus, one attempts to simulate spectral value curves whose maximums respectively lie in the red, green and blue spectral range--similar to the curves in FIGS. 6 and 7. However, the required, negative spectral sensitivities are left out of consideration. Consequently, no color reproduction system has spectral value or color mixing curves that exactly coincide with the linear combination of the cone sensitivities of the eye.
The spectral sensitivity curves of various apparatus also differ greatly. Even apparatus of the same type can have channel sensitivities that depart greatly from one another. No current, technical apparatus is thus in the position to implement and exact color analysis according to the three-range method. Color images that are reproduced with various systems thus end up different with respect to their color reproduction, even on the very basis of the different color analysis. Electronic manipulations performed at the color value signals can in fact lead to an improvement of the color analysis, when, for example metameric problems can be precluded when scanning originals that have already been reproduced. Since reproductions--diapositives, reflected light images or even color prints--are continuously constructed from three colorant constituents known in terms of their spectral composition according to what are as a rule known as mixing laws, exactly one spectral transmissivity or remissivity exists for each triad of color densities or colorant concentrations. Given knowledge of the spectral sensitivities of a scanner, an unambiguous although non-linear relationship between the colorant concentrations in the original and the color value signals at the output of the scanner can be recited. Given a known type of original and known scanner, an unambiguous and reversible relationship between the color values in the original and the color values derived therefrom with this one scanner exists by the calculatable spectral transmissivity or remissivity. An exact allocation between scan values on the one hand and exact color values on the other hand is thus possible via a three dimensional table. Given different combinations composed of the type of original and scanner, however, different relationships apply, and thus different function tables apply. In practice, however, one is limited to simple manipulations such as non-linear pre-distortions and linear metricizing of these scanned signals in order to achieve an improvement. Exact allocations between scan values and color values, however, cannot be achieved in this way. The required apparatus adjustments (non-linear characteristics and matrix coefficients) can in fact be computationally optimized. In practice, however, the method cannot be implemented since the spectral properties of the image originals are usually unknown to the user. One is merely limited to a correct balancinc of the gray or achromatic axis in order to obtain a harmonic image impression. F. W. Vorhagen, Ueber die farbvalenzmetrische Optimierung der Fabwiedergabeeigenschaften elektronischer Reproduktionssysteme, Dissertation, RWTH-Aauchen, 1978.
Given such a correction, of course, all components of the color image scanning system dependent on the wavelength must also be computationally taken into consideration, such as, for example, the spectral distribution of the illuminating light source as well as the spectral transmission of the optics and the spectral and locus-dependent sensitivity distribution of the image sensor.
FIG. 8 shows some technically realized, spectral sensitivity curves of video film scanners made by various manufacturers. All curves exhibit substantial differences compared to the rated curves of FIG. 7. As FIG. 9 shows, even the spectral sensitivities of drum scanners made by one manufacturer are likewise not uniform compared to one another. FIG. 10 shows the spectral channel sensitivities of a flat bed scanner from the Desktop Publishing area. This scanner achieves the color separation with three luminescent lamps in the red, green and blue range that light in chronological succession. The line spectra of the lamps can be clearly seen.
The color value signals and the reproductions that, for example, can be produced with the various apparatus from the same image original are just as different as the spectral sensitivity curves of the scanners shown here. The three-range method is thus not suitable for an exact color analysis in reproduction systems. The fact that it is nonetheless currently exclusively employed in technical systems is because it can be realized relatively simply and cost-beneficially. Moreover, all color reproduction systems currently in existence are constructed as what are referred to as "closed systems", wherein color analysis and color synthesis occur in the same apparatus, so that method-associated errors of the color analysis can be partially corrected by manipulations in the color synthesis. The data produced with one apparatus, however, are fundamentally not suited for color synthesis on another apparatus. An "open structure" as will be required in the future in data networks presumes an exactly defined reference interface as a connection between color analysis and color synthesis so that every scanner can produce color-binding linked with any reproduction system. In summary, it can thus be stated that the known color image pickup systems implement the color analysis according to the three-range method in only an imprecise fashion and require exact knowledge about the physical properties of the image original and of the scanner for correcting the electrical signals, this knowledge being usually only available to a person skilled in the art. In future electronic color image systems, however, a color image scanning system must be capable of being operated without knowledge of the inner physical relationships; otherwise, it could not be employed in a network or even in the private domain.